1. Field of the Invention
The present invention relates to coated steel sheets or strips (herein called steel sheets) having a coating which can provide excellent corrosion resistance, particularly corrosion resistance in their worked portions, and useful for applications in automobiles, home electric appliances, and construction.
2. Description of Related Arts
Galvanized steel sheets have long been used widely as a surface treated steel sheet, because they can be commercially produced on a mass-production scale without sacrificing their corrosion resistance before or after paint coating and workability as well as strength inherent to cold rolled steel sheets.
In more recent years, trials have been made using galvanized steel sheets as rust preventive steel sheets in automobiles, particularly in cold regions, for preventing rust caused by salt dispersed on highway roads to prevent freezing of the roads. The trend is, however, that more and more demands are being made for satisfactory corrosion resistance of the galvanized steel sheets under severe corrosive environments.
As the means for meeting with the demand for improved corrosion resistance of the galvanized steel sheets, it has been known and commonly practiced to increase the amount of the zinc coating itself and in addition to the increased zinc coating amount, many various proposals have been made for alloy coatings as a means of inhibiting the dissolution of the zinc coating itself. Most of these proposed alloy coatings contain iron-group metals such as Fe, Ni and Co as the alloying element.
The steel sheets electro-plated with a zinc iron-group-metal coating as disclosed in Japanese Patent Publications 50-29821 and 57-61831, for example, are characterized by their excellent corrosion resistance before and after paint coating and have been successful in commercial production and application. However, a strong demand still exists for further improvement of their corrosion resistance.
Further, steel sheets coated with a zinc or zinc alloy coating containing chromium have been proposed as disclosed in Japanese Patent Publications 59-38313 and 59-40234. and Japanese Laid-Open Patent Applications 61-130498, 61-270398 and 62-54099.
All of these proposed chromium-containing coatings contain a very small amount of chromium, which produces only an auxiliary effect on corrosion resistance. Therefore, for the purpose of definitely improving the corrosion resistance, it has long been desired to precipitate a larger amount of chromium in the coating.
For applications in automobiles and home electric appliances, severer demands are being made for improved surface appearance in addition to corrosion resistance, and to meet with the demands, an appropriate treatment for the surface brightening of coatings is strongly desired.
Up to now, no successful art is known for increasing the chromium content in the zinc coating and no successful art is known for producing Zn-Cr coatings with a high-chromium content and yet excellent in the surface appearance.
Thus it is impossible to obtain a satisfactory coating having good surface brightness and workability merely by increasing the Cr.sup.3+ ion concentration in the plating bath. The increased chromium ion concentration causes various hindrances in operation, such as a sharp lowering in the current efficiency, which prohibit a commercial production of steel sheets having a high-chromium alloy coating.
Meanwhile, to meet with the demands for improved corrosion resistance for applications in automobiles, in particular, a complex coated steel sheet has been developed by subjecting the metallic primer coating to a chromating treatment and then applying an organic coating thereon.
In the art of these steel sheets having the complex coating, main considerations have been given to improvements of the paint composition which forms the upper-most organic coating, and inevitably, no full satisfaction has been achieved concerning improvements of corrosion resistance, press formability, spot weldability, etc. For example, a zinc-rich paint coated steel sheet shows inferior press formability and is not satisfactory with respect to corrosion resistance and weldability, and a steel sheet coated with a paint containing electric conductive pigments is not wholly satisfactory despite its improved press formability and weldability. Further, in all types of the organic coated steel sheets mentioned above, the organic coating is applied in a relatively large quantity exceeding 5 .mu. in thickness which is rather detrimental to press formability and weldability.
More recently, steel sheets having a metallic-organic complex coating also have been developed in which the organic coating is applied in a relatively small quantity of less than 5 .mu. in thickness. For these thin-type organic coatings, trials have been made to improve the corrosion resistance by introducing rust preventive pigments in the organic coatings. For example, Japanese Laid-Open Patent Application 59-162278 discloses an organic coating in which chromium compounds as the rust preventive pigments are added to a water-dispersion type emulsion resin, and Japanese Laid-Open Patent Application 60-50181 discloses an organic coating in which silica is added as the rust preventive pigment. However, these trials have been found unsuccessful in improving the corrosion resistance.
The corrosion resistance of the steel sheets having a metallic-organic complex coating mentioned above is imparted mainly by the organic coating. However, as the thickness of the organic coatings is required to be thinner from the viewpoint of press formability and weldability, improvement of the metallic primer coating itself is required from the viewpoint of corrosion resistance.
More specifically, as the metallic primer coating, Zn coating, Zn Ni coating, Zn-Fe coating and the like are applied, and in the thin-type organic coatings, the organic coating is further thinned when subjected to the press forming and slight damage or scratches can easily penetrate the thinned organic coating to reach the metallic primer coating or even to the substrate steel sheet, thus causing local exposure of the metallic coating of the substrate sheet. Therefore, the corrosion resistance of these coated sheets must more and more rely on the corrosion resistance of the metallic primer coating alone. However, the conventional metallic coatings cannot impart satisfactory corrosion resistance, and are not reliable for maintaining good corrosion resistance after press forming.